Sub Slab Insulation - EPS vs XPS

Update November 2017

Since first writing this posting, my viewpoints have updated based on additional gained knowledge.  While I still believe that XPS wets up slower than EPS, I now know that both will wet up in the long run in damp environments. So drainage below (sub-slab) or along (vertical foundation) is key to keeping these products performing well. 

When choosing between the two products, I also agree with the recommendation by some to just increase the thickness of EPS by  20% to account for R value loss when wet.  This is based on the fact that EPS currently has a much better environmental footprint over XPS.

And indeed this was the direction I had planned to take on the house I am building.  But then I found out that ROXUL had approved its mineral wool insulation for sub slab installations.  This represents an even better alternative to rigid foams.  Mineral wool is free draining, has a smaller environmental footprint (especially ROXUL that is produced using electricity from a nearby Hydro Electric dam), and is hated by insects and rodents (relevant for vertical installation on the outside of a foundation).   ROXUL recommends their ComfortBoard 110 product for this application. 

While I now plan to use this product below my slab, I still feel that long term unbiased testing of the typical sub slab insulation options would still be of value to the building industry.  This is why my house currently under construction will now include a sub-slab lab comparing XPS, EPS, and ROXUL.  We will look at wet-up, R value loss, and compression of these insulation's over many years under real world conditions.  The slab will include removable panels allowing access to the insulation below.  Details for the lab can be viewed at theEnclosure.ca


 
Original Post

As some of my regular readers know, I tested samples of EPS and XPS in an underground wet environment to see which over time absorbed more moisture.

I described the experiment design in my blog posting of Aug 22, 2013 and describe the start of the experiment in my posting of October 6, 2013.

Fig 1: Samples at beginning of experiment.  These were buried below aprox 4 ft of dirt in a wet environment subjected to regular/constant ground water.

 I dug up the samples March 25, 2014 and the results do not look good for EPS.

Table 1: Weight of buried samples at end of 9 months.

As you can see in table 1, over the same period of time and in the same conditions, EPS absorbed an average of 258% of its original mass in additional water compared to only 31% for XPS.

Once I finished my on-site testing of the samples, I then took them all down to Fitsum Tariku, an instructor at BCIT and Director of Building Science Centre of Excellence (to name just some of his many accomplishments and titles). Fitsum offered to have some of his Masters students in the Master of Engineering in Building Science program run some experiments to determine the total moisture take-up potential of both products as well as the thermal resistance once saturated.

Unfortunately they were unable to use my buried samples because they were too damaged (I should have bed them in a thicker layer of sand both below and above to protect the integrity of the samples - however it was still a very revealing test based on my results in table 1 above).  Instead they used samples I had submerged in a tub of water and others I had on a shelf during the experiment.

In the following tables, you can see that EPS also does poorly from a R-Value retention point of view when saturated compared to XPS.

Table 2: Dry weight of samples measured by BCIT

Table 3: Measured R-Value (using Hot Box) of both dry and wet samples

Table 4: Difference in R-Value between two insulation types both when dry and wet.

Table 5: Loss of thermal resistance when saturated.

The last graphic tells it all - EPS looses 15.7% of its thermal resistance when in a wet environment and saturated compared to only 3% for XPS.

So why is EPS used in many 'green' projects.  This stems from the EPS industries claims that it represents a lower Global Warming Potential vs XPS due to its use of Pentane as a blowing agent compared to the traditional HCFC agent used by the XPS industry.  But XPS manufacturers like Owens Corning have already replaced their blowing agent with a Zero Ozone Depleting formula.

Finally, one positive recorded result is that both products met or exceeded their published thermal resistance per inch of R4.27 for EPS and R5 for XPS (as shown in table 3 - dry state). 

The outcome in our view is pretty clear cut - over the extended period representing the lifespan of a dwelling (50+ Years), the lower initial thermal resistance, and then the significant deteriorating of R value if EPS gets wet and stays wet, far out-way any environmental benefits claimed for EPS.  The obvious choice for below slab insulation applications is clearly XPS when all factors are taken into consideration.

Sample Specifications:
XPS - Owens Corning Foamular C-300 (30 psi) 
EPS - Plasti-Fab PlastiSpan 30 (30 psi) 

SENWiEco concludes testing of DURISOL ICF Block

When choosing a foundation your options are typically a site formed and poured concrete wall or some form of insulated concrete form (ICF) wall.  Early on in the process I gravitated to an ICF wall because it would eliminate the need to hire forming crews and rent and fabricate forms.

When looking at ICF, the traditional product is made from some form of EPS foam which has a very high embodied energy, lots of off-gassing, and is made from non-renewable components. The foam industry (EPS and XPS) will try to 'green-wash' this by stating the foam, as an insulation, reduces heat loss and reduces carbon output over the lifespan of the dwelling.  Yes this is true for ANY insulation, so choosing an insulation with a starting lower embodied energy will put you that much further ahead on your reduction goals. So again, early in the process I looked for a product that on the surface was friendlier to the planet.

One of the benefits of all ICF walls is that they typically require a smaller concrete core than a standard foundation.  The code allows for a 5.5" core on ICF walls where a standard site formed wall generally start at 8".  The reason for this escapes me because the ICF product itself is not considered structural so why would all walls not be allowed to be only 5.5" regardless of forming method.  If someone knows the answer to this please post a comment.  The smaller core of the ICF significantly reduces the concrete needed and therefore the cost and embodied energy of the overall wall.

One of the other downsides to a typical ICF forming material (foam), is that you end up with too much insulation on the inboard face of the core.  This decouples the core from the interior environment and can lead to condensation in some isolated cases, but more importantly it limits the walls ability to be a moderating force to the homes inside environment. An exposed concrete wall can buffer the temperatures by acting as a thermal mass.

The further downside to foam style ICF blocks is that just about everyone loves them from rats to ants.  They burrow and nest in the product creating holes in your thermal blanket.  They are also quite fragile and can be easily damaged during construction and require significant blocking during pouring to prevent blow-out.

My quest for the perfect block led me to the Durisol product.  It is made with virgin but scrap wood (manufacturing waste and tree tops).  This wood is chipped and then through a patented process, the organics are removed to create a mineralized wood fibre (think petrified wood).  This is then added to a cement slurry and formed into the ICF block.  This process and product would help meet my goals to dramatically reduce the embodied energy of the foundation.

There is another similar product made by Faswall, but my research indicated that this product utilized non-virgin wood sources like used pallets and had a lot more dimensional tolerance issues with the block itself.  I also was informed that Faswall was initially going to be a licensee of Durisol but ended up swiping the formulation and heading out n their own.  This did not sound like the right fit for me so I focused on Durisol even though it meant I would have to freight them from back east.

Once I decided to seriously consider Durisol, I then wanted to ensure it was suitable for the task. My immediate concern was that the blocks would rot.  But the product has been used for decades as sound abatement walls on highways (where some of the wall is always buried) and I received a letter from the Ontario Ministry of Transport advising that they have never had to repair a wall due to decay (just traffic accident damage).

My next concern was how would this wall act from the point of view of air and moisture movement.  It was made clear from the beginning, that I would need a independent air barrier as this product was air permeable (it has webs that penetrate through the concrete core so the core is not continuous).   So this was a negative against the product when compared to foam, but as I wanted a bullet proof building enclosure, I had always planned on an robust WRB (water resistant barrier) on the exterior of the foundation.  I think the idea of 'damp-proofing' a foundation wall in a rain forest climate is ludicrous and had always planned on Water Proofing my wall.  And a waterproof membrane is almost always also an air barrier.

My next concern was how the blocks would act if subjected to regular wetting.  The manufacturer claimed the product was unable to support capillary action and had some university testing to support.  But I was not satisfied and so set out to torture test the product over 16 months.  I started the experiment in Jan of 2013 (Begin experiment).  At the eight month mark I posted the status) Status at 8 months) and then altered the block to also contain the concrete core.  The experiment concluded on June 1, 2014.




All off my testing supported the manufactures claims.  This was a free draining assembly that did not support moisture movement from the outboard to inboard face.   I will also be preventing moisture movement through the footings via a FastFoot mebrane and also using a touch-on or self-adhered AB/WRB mebrane on the outside face of the foundation and so will have a very durable and forgiving assembly.  I now felt confident using this product on my project and have now received the product on site.  Once the excavation is complete, I will post some videos on the installation of the product (visit my project journal for the tribulations in getting these goods to site).

As I have time (may be at end of construction, I will also try to post some cost comparisons between the various options and the embodied energy numbers).

Thanks for visiting.